MEMOIR OP BARON HALLER. 51 
pears, the yolk and intestines are included in the 
abdominal cavity by means of the acquired irritability 
of the muscles which cover that cavity, and this 
animal with a double body becomes a common 
chicken. So is it in another instance already no- 
ticed; the heart becomes that well defined organ, 
instead of being a half ring, separated widely from 
the spine and placed almost without the chest. It 
is the cellular membrane, passing from the fluid state 
to a state of considerable solidity, which draws the 
separated portions of the heart towards each other, 
and approximates the whole to the hack hone ; and 
similar causes mould the chick, and hind it upon 
itself, till it attains that perfection in which we 
find it. 
“ I believe,” continues the author, “ enough has 
now been said to vindicate my opinion concerning 
the doctrine of gradual evolution. The probability 
appears to he, that all the essential parts of the 
chick exist throughout all time ; not indeed such 
as they appear in the adult animal, but so disposed 
that certain and provided causes hastening the in- 
crease of some of these parts, hindering that of 
others, changing their relative places, making ma- 
nifest organs which were formerly transparent, and 
giving consistence to fluids, in the end form an 
animal very different from the embryo, but in 
which no part exists which had not essentially 
existed before. This is my explanation of develope- 
ment.” 
These observations lead to reflections not less 
